UNICODE_STRING - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _PHNT_NTDEF_H
#ifndef _NTDEF_

typedef struct _UNICODE_STRING
{
    USHORT Length;
    USHORT MaximumLength;
    _Field_size_bytes_part_opt_(MaximumLength, Length) PWCH Buffer;
} UNICODE_STRING, *PUNICODE_STRING;

#endif
#endif

View code on GitHub

This structure is documented in Windows Driver Kit.